As Fig. 1a, the emission spectrum of tryptophan at 280 nm excitation
showed a maximum at 360 nm due to the intense native
fluorescence of the indole-moiety of tryptophan. After reacting
with OPA, the emission maximum shifted to 455 nm, which
revealed the fluorescence resonance energy transfer (FRET) from
the intrinsic indole moiety to the isoindole formed with OPA. In
addition to the 340 nm peak of primary amines, the excitation
spectra of OPA-tryptophan adduct (Fig. 1b) exhibit another dosedependent
emission maximum at 280 nm, which can be used to
selectively quantify tryptophan. Moreover, the emission spectrum
of tryptophan shows a broad overlap (65%) with the excitation
spectrum of its OPA-adduct (Fig. 1c), which leads to a high energy
transfer efficiency and therefore a high sensitivity in tryptophan
measurement based on FRET.